Clarifying the three-dimensional structure of a biological substance such as a protein or a nucleic acid is very beneficial from an industrial viewpoint. More specifically, for example, it reveals the functions of the biological substance in vivo, thus allowing efficient drug development.
Among various methods for analyzing the three-dimensional structure of a protein, nucleic acid, or the like, for example, crystal structure analysis, especially by X-ray diffraction imaging, is highly effective and has been used widely. To perform analysis by this method, it is necessary to produce crystals of an analyte (a substance to be analyzed), i.e., the protein, nucleic acid, or the like. To this end, there has been used a method for precipitating crystals of (i.e., crystallizing) the analyte out of a solution containing the analyte (Patent Document 1 etc.). However, precipitating crystals of (i.e., crystallizing) a protein, nucleic acid, or the like out of a solution thereof is difficult and requires very advanced techniques. The reason for this is considered to be that, in the solution of the protein, nucleic acid, or the like, crystal nucleus formation does not occur, or even if it does, the formed crystal nuclei disappear right away.
As a method for obtaining crystals of a protein or a nucleic acid more easily, there has been used a method for crystallizing the protein or nucleic acid in a gel, instead of in a solution (Patent Document 2). However, in order to obtain crystals excellent in quality, strength, etc. more easily, further improvement in the techniques has been demanded.
Under these circumstances, there has been an attempt to work on crystallization of a protein by irradiating a solution of the protein with a laser beam to cause formation of crystal nuclei of the protein (Non-Patent Document 1). The principle of this method can be explained as follows. First, when the protein solution is irradiated with a laser beam, a phenomenon (so-called cavitation) occurs in which bubbles are formed and then disappear within a short time. When the formed bubble expands, the protein in the solution is adsorbed onto the surface of the bubble. Then, when the bubble contracts in the course of disappearing, the surface area of the bubble becomes smaller, so that the adsorbed protein is concentrated, resulting in an increased protein concentration (density). It is considered that crystal nuclei are more likely to be formed at this portion where the concentration of the protein is high (the protein is concentrated).
Furthermore, it has been revealed that crystal nucleus formation can be induced more efficiently by irradiating a protein solution with a laser beam after the viscosity of the protein solution has been increased or the protein solution has been turned into a gel (Non-Patent Document 2).